[exim] how to discard return emails when they are spams?

Dear all,

My question is simple but I could not find any answer.

When a spammer sends an email to any address of our domain and that the
mailer daemon detects it is a spam, it returns the mail to the From address
telling it won't be delivered.

I'd like to be able to discard the message without any reject mail.
Actually by returning the mail it sends a spam to someone, and as a
consequence our domain may be blacklisted.

I hope this is understandable

All the best

Jacques